For example, JP H10-48180 A, which corresponds to U.S. Pat. No. 6,120,677, describes a technology for detecting the impedance of an element (element impedance) of an oxygen concentration sensor that is used for controlling an air fuel ratio of an internal combustion engine.
In the technology of JP H10-48180 A, a constant current for measuring an impedance, that is, for measuring a resistance value is applied to a cell as the element only for a fixed period of time. A difference between a both-end voltage of the element before being applied with the constant current and a both-end voltage of the element when being applied with the constant current is detected as a value that is correlated with the impedance of the element. The difference corresponds to the amount of change of the both-end voltage due to application of the constant current.
In the technology of JP H10-48180 A, after the constant current for measuring the resistance value is applied to the cell, a constant current having an opposite polarity is applied to the cell only for the same fixed period of time so as to shorten the time to return the cell to a normal state for detecting the air fuel ratio.